If they exploded, the TSA and local bomb squads around the country would have their hands full, that's all I can say.

They're about as likely to explode in normal use as your car, which is to say they're highly unlikely to explode.

All batteries can be forced into conditions where they'll be unsafe. Here's a video of a lithium ion battery exploding after being intentionally forced into an unstable, highly dangerous state. Normal usage isn't going to do this, and recalls were issued in the past for faulty Li ion batteries across a wide variety of manufacturers, including Apple.